In a carburetor type internal combustion engine, so-called "dieseling" is a phenomenon wherein the engine continues to rotate after the ignition switch is opened because of compressed ignition of fuel issued from the idle port of the carburetor. In such an engine, dieseling is generated under condition where the temperature of intake air is relatively high. Such dieseling causes a large amount of unburnt air-fuel mixture to be provided in the exhaust manifold. This large amount of unburnt air-fuel mixture causes overheating of the catalytic converter arranged in the exhaust pipe of the engine.
To prevent dieseling, an engine has been already provided which has an electro-magnetic value situated on a primary low speed fuel passageway of the carburetor, connecting the idle port with the float chamber of the carburetor. The electro-magnetic valve operates to close the passageway, when the ignition switch is opened to stop the engine, in order to prevent the fuel from being transmitted to the idle port. Thus, dieseling caused by fuel from the idle port does not occur.
However, the above-mentioned known art cannot prevent dieseling caused by the fuel from the secondary low-speed fuel passageway of the carburetor. Such dieseling occurs when the secondary throttle valve of the carburetor is, after the ignition switch is opened, not completely closed due to clogging in such a manner that the secondary throttle valve is located above a slow port connected to the float chamber via the secondary low-speed fuel passageway.